During ovulation increased concentrations of LH are found in the body fluid of the woman. Before ovulation the concentration of the LH in urine is about 25 -100 I.U./Liter which rises to a peak of about 150 -600 I.U./Liter at the time of ovulation. By testing urine samples daily between two menstrual periods it is possible to determine the day on which the LH concentration rises to its peak and thus, the day of ovulation, i.e. the day on which the woman is fertile.
It has been found that the concentration of the LH can be estimated by means of an immunoassay using HCG as standard because the two hormones cross-react serologically. Thus, one I.U. of HCG = 2.5 I.U. of LH. The present invention makes use of this cross-reactivity. Erythrocytes sensitized with HCG are agglutinated with an anti-HCG serum. The hemagglutination is inhibited in the presence of free HCG beginning with a predetermined concentration thereof in the body fluid or in the presence of an equivalent concentration of LH.
This predetermined concentration is a function of the sensitivity of the test. The sensitivity can be adjusted to any desired value by altering the concentration of the reactant. It has been found that, for the purpose of the present invention, by adjusting the reactants so that hemagglutination is inhibited starting with a concentration of LH equivalent to 50 I.U./Liter of free HCG in the body fluid, it is possible to determine the day of ovulation, which is the day on which the woman is most fertile.